Data networks are, inter alia, used for networking devices in automation technology. Such data networks are also referred to as field bus systems and are, inter alia, standardized in IEC Standard IEC 61158.
In automation technology, sensor/actuator level devices such as I/O modules, measuring transducers, drives, valves and user terminals, inter alia, are connected with control level devices, such as industry PCs, via the data network. In order to allow process control in industrial manufacturing, real-time communication of the network users is required, where the data transmission is completed within predefined periods of time. For a high-performance data network in automation technology, a high data transmission rate and a low error rate during data transmission are also required.
Data networks in automation technology are frequently based on the Ethernet standard. An Ethernet-based field bus system that allows real-time communication is for example the EtherCAT fieldbus according to IEC 61158.
As a rule, data networks in automation technology are designed as master-slave networks, in which one or more network users as masters control the data exchange in the data network. The remaining network users are passive network users and exchange payload with a master only upon request by the latter. These so-called slaves receive payload sent by the master and send, upon request by the master, payload directed to the latter.
As a rule, the data exchange is realized by means of data telegrams. Apart from the payload, these include additional control data. The control data may here include the type of telegram, the addresses of the targeted network users and/or a checksum. This control data is used to control the distribution of the data telegram via the data network and the processing of the data telegrams by the network users. Moreover, the data telegrams may have one or more access indicators. These may represent for example the type and the number of the data accesses to the data telegram that were carried out.
The network users may be located within the data network for example in a ring topology or in an open ring topology, which is referred to as linear topology. In a master-slave network in linear topology, the data telegrams pass, starting from the master, through all the network users of the data network in order and are subsequently returned to the master. In the course of this, each individual data telegram passes through all the network users at least twice.
A master-slave network in linear topology may also include branching points, where a network distributor may be located that is connected to three or more data paths. If the network distributor receives a data telegram from a master via a first data path, it will forward the data telegram, if needed after processing, onto one of the other data paths, where the data telegram passes through all the network users connected to this data path, so that they can access the payload of the data telegram, and subsequently the data telegram is sent back again to the network distributor.
Once the data telegram has been received again, the network distributor forwards the data telegram for data exchange to the network users of the next data path. Once all the data paths have been served in this way, the network distributor finally sends the data telegram back to the master via the data path that establishes the link to the master. In such a branched data network in linear topology, a data telegram also passes successively through all the network users. In the course of this, it passes through all the network users at least twice, through the network distributor at the branching point at least three times.
In the case of a master-slave network in linear topology, the run time of the data telegram may become rather long, especially if a large number of network users are connected to the data network.